Plant photosynthesis relies on energy in the visible light spectrum. The excitement of electrons in plants' leaves is activated by energy in the form of visible light from the sun. This electron energy is converted into Adenosine TriphosPhate (ATP) and powers cellular respiration, i.e., the metabolic reactions and processes that take place in a cell or across the cell membrane to get biochemical energy from fuel molecules and the release of the cells' waste products.
The amount of direct sunlight needed to power respiration varies greatly from plant to plant. While many plants require up to six hours of direct sunlight a day during the growing season to thrive, the same light energy may harm a different species of plant, causing its failure. Knowing the difference in sunlight requirements is a basic component to successful gardening. Typically, a gardener will draw a map of their site, watch the sun pass throughout the course of a day, and make a diagram or rough sketch. However, this process is time consuming and inaccurate.
U.S. Pat. No. 3,779,651 to Gunlock, teaches the use of light-sensitive material having a filtered cover. A color change in the light-sensitive material relates to a reference color that has been printed on the substrate. Gunlock requires the use of a color reference for comparison of the activated material to determine the quantity of radiation which has been received. A legend is used to identify by color plants which may be suited for growing in the subject light environment. Gunlock also discloses a disc that has a small portion exposed in a window, so that a segment of the disc can be rotated into the exposure window position. Thus, a reference color is adjacent to the exposed disc area on a non-rotating surface. Gunlock also teaches the use of strips that may be exposed under a filter and placed adjacent to the printed reference colors.
Sunlight meters such as that disclosed in Gunlock have multiple component parts that must be manufactured (increasing cost) and assembled by a user (increasing complexity). Moreover, Gunlock is limited in that colors can only be analyzed with reference to reference color charts. Thus, such meters are subject to user error and costly to manufacture.
U.S. Pat. No. 4,168,124 to Pizzi describes the use of electronic sensors to detect and quantify the level of sunlight over a period of time. However, that disclosed methodology, while yielding accuracy, is too complex for everyday use for a large number of applications.
U.S. Pat. No. 2,337,534 to Barber similarly teaches a disc or sheet of photosensitive paper that darkens when exposed to light for monitoring exposure of book pages. However, Barber's invention is suitable for brief exposures lasting only a few seconds, and an exposure level is determined by the intensity of the blackening of the photographic light sensitive paper. As such, utilization of the Barber device is measuring light intensity rather than duration.
U.S. Pat. No. 6,132,681 to Faran et al. similarly teaches a disposable dosimeter for measuring human exposure to ultraviolet radiation. Although Faran teaches the use of at least one active chemical for color change measurement at higher temperatures, Faran is disadvantageous as it is measuring human exposure to ultraviolet radiation.